Jet exhaust noise results from the turbulent mixing of exhaust gases with the atmosphere. The noise is influenced by the shearing action caused by the relative speed and temperature between these airflows. This noise can be reduced by mixing these two airstreams internally and/or by reducing their relative temperature and velocity. Supersonic airplanes require a propulsion system that produces high specific thrust (high exhaust velocities) for acceleration and cruise at supersonic speeds. Therefore, the engines for supersonic airplanes are straight jets or very low bypass ratio engines. To produce sufficient thrust for take-off, these engines have to be operated at or near their maximum power capability, producing high exhaust velocities. Therefore, the exhaust noise is especially high and hard to reduce on supersonic airplanes.
Ejector nozzles are currently used in pure jet and low bypass ratio engines as noise suppression devices. Generally, ambient air is introduced (i.e., aspirated) into a nozzle duct through auxiliary inlets. The ambient air mixes with the high velocity and hot engine exhaust, thereby reducing the overall velocity and temperature of the engine exhaust before it exits the nozzle. Mixing devices are generally used in conjunction with the ejectors in the pure jet and low bypass ratio applications in order to more thoroughly mix ambient air with exhaust gas.
Turbojet and low bypass ratio engines require high aspiration levels (i.e., generally greater than about 60% of the engine exhaust) in order to provide adequate levels of noise suppression. High aspiration levels require the ability to significantly vary nozzle geometry. This requires ejectors capable of assuming a wide range of positions. In addition, the ejectors and mixing components must be capable of being selectively removed from the nozzle duct airflow path in order to transition the nozzle to an acceptable performance configuration for high speed flight, when noise suppression is not required. The combination of these requirements often results in nozzle designs that are heavy, complex, and have poor performance characteristics.
In contrast to the pure jet and low bypass ratio engines, intermediate bypass ratio engines (i.e., generally in the range of 0.6 to 1.2) usually create less jet exhaust noise to begin with due to their ability to produce thrust with lower average exhaust velocities. Noise reduction for intermediate bypass ratio engines generally consists of using a common or integrated exhaust nozzle that partially mixes the bypass and primary exhaust gases prior to their ejection into the atmosphere.
It is known to use ejector nozzles to improve nozzle performance in specific flight conditions. In particular, adding ambient air around the periphery of the exhaust gases of a straight jet or low bypass engine reduces aerodynamic boattail drag at transonic conditions. This is done during transonic and supersonic flight conditions where noise suppression is of no concern. For low bypass ratio applications, ejector nozzles are used to reduce jet noise. But this has generally only been done as a retrofit to older subsonic airplanes to bring them in compliance with new and stricter noise rules. It has been proven repeatedly that an ejector does not provide performance benefits at low speed operation such as take-off and landing. Ejectors may provide a small thrust augmentation statically, if well designed. But at typical take-off speeds the thrust augmentation has been eliminated by external drag and internal losses. At subsonic cruise speeds a deployed or fixed geometry ejector causes significant thrust losses and is absolutely useless, since noise reduction is not needed in cruise.
Up to very recently, it was a common belief that the optimum engine cycle for supersonic airplanes are straight jets or very low bypass ratio engines. In order to meet stage 3 or more stringent future noise rules these engines needed jet noise suppressers with a capability of up to 22 decibel. This called for ejector nozzles with aspiration ratios of 80 to 120%. The huge geometric variation required cannot be met by an axisymmetric exhaust system, but required two-dimensional ejector nozzles of large dimensions, high complexity and high weight. Also, the high exhaust gas temperatures of these engines make the material selection for those exhaust systems very difficult.
Newer engine optimization studies indicate that when engines are treated to the same low jet noise level, the engine cycle for supersonic airplanes optimizes at significantly higher bypass ratios (0.8 or even higher). Since the engines for supersonic airplanes are generally sized by end-of climb thrust requirements, they produce excess thrust at low speeds. They can therefore be operated at less than full power for take-off. Increased bypass ratio and the associated part power take-off both contribute to lowering jet velocities and the requirements for jet noise reduction with the help of an ejector nozzle. The increased bypass ratio also lowers the mixed flow gas temperature and makes the materials choice for the exhaust system simpler.
These discoveries suggest that an optimum noise suppression solution for some supersonic aircraft is to aspirate a relatively small amount of ambient air into the engine exhaust of intermediate bypass ratio turbofan engines during takeoff climb-out and landing conditions. This has resulted in a need for such an ejector nozzle. The ideal nozzle should be light, compact, simple, and reliable. The ideal nozzle should include mixing components to further increase the mixing of ambient air with exhaust gas in a way that does not adversely affect the engine and nozzle performance. Also, the ejector nozzle should be operable and efficient at speeds up to Mach 0.7 or 0.8 to permit jet noise suppression during climb-out. Further, the ideal nozzle should not adversely affect the nozzle performance during nonsuppression flight at high speeds.
The efficient operation of an ejector depends on several parameters. First, the internal high energy (pressure) engine exhaust flow has to be accelerated to a very high velocity, so that its pressure drops and helps suck in low energy (pressure) ambient air. Secondly, to maximize the amount of ambient air sucked in through a given inlet passage, it is also important to increase the length of shear layer between the streams, which calls for a lobed confluence of the flows or alternating channels. Thirdly, in order to get as complete mixing as possible inside the limited length nozzle, to treat and contain the associated mixing noise, the cross-sectional size of the interspersed flows has to be small. The present invention is directed to provide an ejection nozzle that meets the afore listed objectives.